This invention concerns a power tool, in particular a hand-held compressed air screwdriver with a compressed air motor, a switch which controls the compressed air supply, and a shutoff valve arranged between the switch in the compressed air supply passage to the motor.
In a known tool of this kind as disclosed in German Opened Specification No. DT-OS 24 23 300, a switch and a shutoff valve are provided wherein a servo valve is connected in series to the shutoff valve and disconnects the shutoff valve by means of a circuit breaking air impulse. This indirect disconnection of the shutoff valve results in a delay in the motor disconnection with all the disadvantages relative to the accuracy of the torque value to be applied to the fastener.
According to this invention, however, a tool of the general type mentioned is provided whereby upon reaching a predetermined torque limit an immediate cutoff of compressed air to the motor is possible which produces high accuracy in adhering to the predetermined torque limit.
According to the invention, this can be achieved in a tool of the type mentioned at the beginning wherein the shutoff valve comprises two coaxial parts which are axially movable independent of one another. One part forms a reset element and the other part forms a shutoff element. The valve elements are held in a starting position by flexible support of the reset element while the compressed air supply is shut off by means of a separate shutoff valve or switch. Upon introduction of the compressed air supply by means of the switch and simultaneous locking of the shutoff element in an open position, which may be the same as its starting position, the reset element is movable away from the shutoff element into a position whereby the fluid shutoff or closed position is achieved, at a predetermined torque or angle of rotation, by releasing the shutoff element and moving the same in the direction of the compressed air flow. The shutoff element may then be moved automatically by means of the reset element back to its starting position from its closed position after shutoff of the compressed air supply by means of the switch.
The development according to the invention makes possible with only one valve, the shutoff valve, the direct control of the compressed air supply to the motor with a particularly high accuracy so that upon reaching the torque limit, motor shutoff is immediately effected. The accuracy of torque control with the tool design according to the invention is also particularly high because the shutoff element is moved into its closed position in the direction of the compressed air flow so that the compressed air flow pushes the shutoff valve into this position. By proper selection of the cross-section area of the upstream and the downstream sides of the shutoff element a pressure difference results which also contributes to a rapid movement of the shutoff element upon release of the locking means.
According to the present invention, the closing speed of the shutoff valve can be increased still more if the shutoff element is elastically biased in the closing direction, that is toward the reset element, which bias produces a high initial acceleration of the shutoff element. In this regard, the shutoff element is also formed preferably by a valve disk of small dimensions. An actuating rod can be associated with the shutoff element which is mechanically or magnetically lockable and wherein magnetic locking can be achieved in simple ways by means of a solenoid or magnetic coil. Mechanical locking can take place by means of an actuating rod which reacts, for example, with a torque coupling which releases the rod after reaching a predetermined torque limit.
The flexible support of the reset element can be provided by a spring or by pressure fluid from the compressed air supply. In regard to the reset element, a cylindrical piston is found to be a suitable embodiment. Moreover, the valve disk may include a resilient ring seal to provide a tight seal at shutoff while the function of the valve disk is, primarily, to guarantee the fastest possible closing upon urging by the compressed air flow after release of the locking means.
Locking of the shutoff element by means of a solenoid is also provided for by the invention. The solenoid may be energized by operation of a miniature switch which includes an actuating member having a resilient switch engaging element. Such a switch actuator enchances the protection of the sensitive miniature switch against excessive loading and comprises an important element of the invention especially for applications in hand-held compressed air screwdrivers since damage to the miniature switch can thus be decreased even with rugged daily use. In a preferred embodiment of the switch, the actuating member engages the miniature switch by means of a spring which is arranged in a hollow shaft portion of the actuating member. The actuating member is supported in the tool housing by means of another spring which urges the actuating member into its starting position. The actuating member reset spring has a smaller deflection range and greater stiffness than the spring engaged with the miniature switch so in this way any overloading of the miniature switch is impossible since, by means of the reset spring, a blocking of the operation path of the actuating member is achievable before the operational spring for the miniature switch goes to the solid condition. The use of an electrical switch makes possible within the limits of the invention not only the immediate actuating of the solenoid for locking the shutoff element but also additional arrangements, particularly the opening of a master intake valve for the pressure fluid and elimination of electronic devices such as are provided for torque and rotational angle measurements.
The present invention still further contemplates that the switch can comprise a second shutoff valve for directly controlling the compressed air supply to the first mentioned shutoff valve. A second shutoff valve according to the present invention may also make possible in its closed position venting at least of the valve chamber of the first shutoff valve so that the shutoff element can be pushed back due to the resilient support of the reset element from its closed position into the starting position.
In yet another refinement of the present invention, the second shutoff valve can also be arranged in such a way that by means of this valve, which turns on the compressed air supply directly to the first shutoff valve disposed downstream, a pressure fluid signal can be conducted to actuate the reset element to move together with the shutoff element of the first shutoff valve back into the starting position of the first shutoff valve.
Further details of the invention are disclosed in the drawings, the detailed description, and the claims.